Some stories resist telling. They are too new, painful or unprocessed. These stories are in charge of their authors (not the other way around). Reading Ruben Gallego's bestselling memoir, White on Black (winner of the 2003 Russian Booker Prize), you would never suppose he was holding anything back. He describes growing up, with severe cerebral palsy, in a series of Russian orphanages and the neglect he and his friends suffered.

He writes with bitter elegance - and keeps his strange, custom-built sense of humour. As a reader, you are mindful of the physical triumph of his having produced a book at all (typed with the index finger of his left hand) and of his miraculous survival. But it is the untold story of his life that is more astonishing even than what he has committed to paper: it is the blackest of fairy tales.

Once upon a time (1968, to be precise) in Moscow, Ruben Gallego was born (his twin died at birth). His grandfather, Ignacio Gallego, was the exiled leader of the Spanish Communist party. Ashamed of Ruben's disability and fearful that it might tarnish his image, he gave his grandson away, at 18 months (during one of the baby's many stays in hospital), to a state orphanage. He lied to his daughter, Aurora, telling her that her baby had died. Somehow, Ruben survived and, with perestroika, was issued with identity papers and able to have a private life. He married (twice), had two daughters, got a job as a computer specialist. Most remarkably, in 2000, he was reunited with his mother with whom he and his grown-up half sister, Anna, now live. This much of the plot I learnt from his publishers. Not a word of it is in his book.

Behind a suburban house, on an industrial estate in Freiburg, Germany, Ruben David Gonzalez Gallego, Aurora Gallego Rodriguez and Anna Yurienen Gallego sit around the kitchen table, like castaways. Aurora, Anna and Ruben live in Freiburg for a practical reason: it has a reputation as a model town for people with disabilities.

Ruben is wonderfully animated, as if his face were determined to make up for everything that cannot move. He uses his hands as much as they will allow, shaking his fingers to make a point and laughing, like a gleeful teacher. His wheelchair is built to his specification: 'Five years' worth of thinking. I hate imitation.'

Aurora pours the tea and interprets in heavily accented English. There is steel in her and a marshmallow softness, too. She smokes exceptionally thin cigarettes. Anna, Ruben's half-sister - young, dark, vivid - is the third point in the triangle.

Ruben begins with a surprise: he explains he never meant to find his mother. He had been told so often that his mother was a 'bitch' at the orphanages that he believed it. But he reasoned: 'OK, but they cannot all be so bad in an entire family.' He made a video about himself and entrusted it to a Russian friend who was travelling to Spain, asking him to pass it on to someone with influence. A Spanish journalist decided to use it as a starting point for a film about a boy's quest for his mother.

The geography of the Gallego narrative shifts restlessly: Moscow, Madrid, Prague, Freiburg. And there are gaps in the story that they seem reluctant to fill. I am still not sure exactly how Aurora was tracked down. But she was in Prague when she received Ruben's letter. 'I was sitting at my workplace [she was a sound technician and journalist at an American radio station] and got a letter in which Ruben was telling me that he was alive and that, if I wanted, I could meet him at Dunkin' Donuts in the evening. Ruben hates Dunkin' Donuts since then... yes, ever since.

'I was in shock, didn't know what to think and so I called Anna and said, "Ruben is not dead. Do you remember I told you about him?" Because I had been telling Anna the story of Ruben since she was little, adapting it to her age. And I said, "You know he is alive and in Prague; what do you think?" She said, "It's great!" She was very enthusiastic and I said, "Is it?" I was in such shock, I didn't know if it was great. But I always followed Anna. So I finished my job, waited two hours and then I went into Dunkin' Donuts.'

Ruben's face lights up as he takes over the story: 'I was sitting in a wheelchair - a very bad wheelchair - and a woman came in. My first question was, "Do you speak English?" And she said, in Russian, "I speak Russian." She was standing very close to me. The contact was instant. I understood who she was. My mother has, like me, been raised partly in children's homes and boarding homes.' He felt they were two of a kind, 'like pilots who meet each other. All my life I had learned to observe and assess people: bad or good, black or white. It's a habit I'll never give up'.

Ruben told her straight away she was being filmed. Given the emotion of the situation, perhaps this didn't matter? Aurora turns on me: 'Yes it did! It did for me. I said, "What?!" Ruben said, "Let's go to the hotel" and I said, "OK" and we went to the hotel and the guys with the cameras behind us were saying, "What happened?" And Ruben told them, "I don't want you." I said, "I want the entire crew, immediately in my home, now. They have to explain themselves; how they can do such a thing?"'

She had had no time to adjust to the knowledge that the son she had believed dead for 30 years had survived. But might her rage with the film crew have been a diversion, easier than reacting to Ruben himself? 'I don't know; I was reacting fast. Behind this was such emotion: sadness and immense pain to see what they had done to my baby - I saw the marks on his body. Pain and then, at the same time, I refused to accept the situation as it was presented to me.'

The film-makers told Ruben: 'You've met your mother; now fly back to Russia.' Aurora was opposed to this. 'No! He wouldn't have made it. He was in such fragile shape; he was dying. I said, "He stays here. You go."' But, first, she gave them what they wanted: 'OK, you brought me my son; I'll give you an interview.'

They got their story, but what began in Dunkin' Donuts with a tearful reunion did not end there. Life isn't as self-contained as a film. Aurora thought it would be easy, that 'everyone would be happy to help' with her son. Instead, she lost her job (her employers disapproved of Ruben). And she had 'only just received treatment for breast cancer'. She laughs. 'But it must not sound like a Mexican soap opera.'

Ruben was about to find his own way of helping. 'I researched the internet to find out how to write a book. And it was not a big help,' he laughs. 'Then I read autobiographies by other writers and this, yes, was helpful. I sorted out the autobiographies of writers whose books paid for a comfortable life from those who became famous or got the money after they were dead; I discarded those,' he laughs again.

He does not conceal his pride in White on Black (now a bestseller in 20 languages). With asperity, he observes: 'When you are in a wheelchair, it is not a good idea to be too modest.' The book has done more than make Ruben comfortable. It has become a talking point in Russia. 'Before perestroika, disabled people just didn't exist. It was not convenient to talk about orphans. Now it is and my book is helping.'

At the end of the book, the acknowlegments are in quaint poetic form and dominated by women (he begins by thanking Eve). He includes the line: 'Thanks to my wives, for having been. Thanks to my daughters, for having been and being.' His estranged wives and his daughters are I presume still in Russia. But an Iron Curtain falls on the subject. He tells me: 'I don't talk about my ex-wives.' I say, lightly, that he is not alone in this. Instead, I ask him about his father. To my surprise, he laughs. He says: 'When I met my mother, she said you are so like your father, except that you are smart.' They giggle at the memory of this. Ruben's father doesn't want to know his son. 'You can live without a father,' he says.

But the question I am longing to ask is about Ruben's grandfather. Can they forgive him? Aurora says she understands what he did, 'but there are things I will never, ever accept in my life'. Ruben offers: 'To forgive somebody, you have to accuse somebody. I am not accusing anybody. My grandfather was a big politician. The more I find out about politicians and their lives, the more I understand my grandfather. But I don't like politicians - none of them. And I don't like my grandfather.'

The love between Ruben and his mother is evident and moving. But it would be glib to insist that this fairy tale has an unequivocally happy ending. Aurora lets on that life, even in Freiburg, is hard. At least Ruben seems happy and, at last, well-fed. He is dazzled by German supermarkets: 'I see bananas, kiwis, pineapples and fruits I do not understand. It is all so beautiful... but I don't need it, it doesn't bring out any emotion in me.' He admits to one exception: 'When I see potatoes, I stop and watch because there are such a lot of them. And everyone else is going by, passing the potatoes without emotion.'

· White on Black is published by John Murray on 16 January. For more information about cerebral palsy, contact the Cerebral Palsy Helpline, which provides free information, advice and initial counselling. Call 0808 800 3333